Please. Help. I think I need to flash a different ROM, but I'm not sure how

Several months ago I rooted it. I followed the instructions here exactly, but to be honest I don't really understand what I did. I did it for the reason most people do... my internal memory was full and it was the only way I could use my external SD card for storing apps, with Link2SD, etc. It's been working fine.

Until now.

the issue is I need to call Virgin Mobile because the cell tower in my town has been down for several days since we had a severe lightning storm. The last time I called them, they wanted to test my phone by putting some codes in. The codes didn't work and I found out after that it was because I rooted my phone and have a different ROM than they put on there.

Now I really need to call them again. But I need to go back to a different ROM that will enable those codes to work when I call them.

I have searched and I can find instructions for rooting and flashing a ROM from the beginning, but not the instructions I need -- how to flash a ROM after i'm already rooted, and also which ROM I should use that would enable me to call in and have those codes work.

I **********really************* would appreciate some help on this. Keep in mind, I'm technically minded but I don't understand rooting and flashing. I might need someone to help me through, step by step.

thanks I will try that! if I return to stock or Froyo Harmonia, what will happen to all the apps that are really on my SD card but being fooled by Link2SD? Will they just disappear and reappear when I go back to the Backside ROM?

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thanks I will try that! if I return to stock or Froyo Harmonia, what will happen to all the apps that are really on my SD card but being fooled by Link2SD? Will they just disappear and reappear when I go back to the Backside ROM?

froyo harmonia is fine. or Rhode, by the same developer.
backup your current Rom in recovery.
then use recovery to format system, data, cache, and boot.
then flash harmonia.
power off. remove your card, you won't need it for virgin's fiddling around.
power on, do their stuff, make sure you're getting data service etc. then reboot recovery, insert card, restore your backup.
reboot. that hopefully should do it.

thanks, it sounds like, from what you're saying, when I backup the ROM, it includes my apps in the backup. so I don't have to worry about where they all go when I change to the other ROM for VM troubleshooting. thanks .